feat: add currency for articles (#834)

Co-authored-by: Jakob Wennberg <jakob.wennberg@gmail.com>

supabase/migrations/20260712140000_currencies_reference_table.sql

@@ -0,0 +1,32 @@
+-- Currencies reference table: single source of truth for the supported
+-- currency code list. Previously the list was hard-coded in many places
+-- (CurrencySchema, the journal-entry/invoice editors, CHECK constraints). This
+-- table lets the UI fetch the options and lets columns FK against it instead of
+-- repeating a literal list. Global reference data, NOT company-scoped.
+CREATE TABLE IF NOT EXISTS public.currencies (
+  code        text PRIMARY KEY CHECK (code ~ '^[A-Z]{3}$'),  -- ISO 4217 alpha-3
+  name        text NOT NULL,
+  sort_order  integer NOT NULL DEFAULT 100,
+  active      boolean NOT NULL DEFAULT true
+);
+
+-- Seed the codes the app already supported. Add rows here (no code change) to
+-- offer more currencies.
+INSERT INTO public.currencies (code, name, sort_order) VALUES
+  ('SEK', 'Swedish krona', 10),
+  ('EUR', 'Euro', 20),
+  ('USD', 'US dollar', 30),
+  ('GBP', 'Pound sterling', 40),
+  ('NOK', 'Norwegian krone', 50),
+  ('DKK', 'Danish krone', 60)
+ON CONFLICT (code) DO NOTHING;
+
+-- Reference data: every authenticated user may read it; nobody writes it from
+-- the app (managed via migrations).
+ALTER TABLE public.currencies ENABLE ROW LEVEL SECURITY;
+CREATE POLICY "authenticated read currencies"
+  ON public.currencies FOR SELECT
+  TO authenticated
+  USING (true);
+
+NOTIFY pgrst, 'reload schema';

supabase/migrations/20260712140100_articles_add_currency.sql

@@ -0,0 +1,14 @@
+-- Articles: optional default currency for the article's price.
+--
+-- Pre-fills the invoice currency when the article is added to a line (the
+-- invoice still carries a single currency; the article supplies the default).
+-- 'SEK' = the existing behaviour. Master data only, never posted; the frozen
+-- invoice line keeps its own currency, so editing this never moves a voucher.
+--
+-- Validity is enforced by a FK to public.currencies, NOT a literal CHECK list,
+-- so the supported set lives in one place (the currencies table).
+ALTER TABLE public.articles
+  ADD COLUMN IF NOT EXISTS currency text NOT NULL DEFAULT 'SEK'
+    REFERENCES public.currencies(code);
+
+NOTIFY pgrst, 'reload schema';